Heathrow Airport has asked all carriers to halve capacity on flights due to arrive on 30 November in the face of potentially crippling strike action by UK Border Agency staff.
In a letter to airlines, Normand Boivin, chief operating officer at Heathrow, said that it is anticipating "serious disruption and delays" of "up to 12 hours" for arriving passengers.
"The delays at immigration are likely to be so long that passengers could not be safely accommodated within the terminals and would need to be held on arriving aircraft.
"This in turn would quickly create gridlock at the airport with no available aircraft parking stands, mass cancellations of departing aircraft and diversions outside the UK for arriving aircraft."
Although it is in talks with the UKBA to find ways of mitigating the effects of the strike, Boivin said "we have not yet been given a contingency plan that gives us assurance we could avoid this scenario".
Instead, the airport is planning a normal flight schedule but is asking airlines to voluntarily reduce load factors on international flights to Heathrow by 50%.
"By following this protocol, airlines will reduce the risk of being diverted to an airport outside the UK and reduce the risk of having an outbound flight cancelled."
BA, which holds the largest number of slots at Heathrow, said it is continuing to plan for a normal schedule on 30 November but is offering customers the opportunity of changing their arrival to dates unaffected by the strike.